Fundamentals of Engineering Materials Science 2 — Questions and Answers
Question 1: On the iron-carbon phase diagram, the eutectoid composition is approximately:
- 0.022 wt% C
- 0.76 wt% C (Correct answer)
- 2.14 wt% C
- 4.30 wt% C
Correct answer: 0.76 wt% C
The eutectoid point occurs at 0.76 wt% C and 727°C, where austenite transforms isothermally to pearlite (ferrite + cementite).
Question 2: Martensite in steel is formed by:
- Slow furnace cooling of austenite
- Rapid quenching of austenite (Correct answer)
- Isothermal holding just below the eutectoid temperature
- Prolonged annealing in the two-phase region
Correct answer: Rapid quenching of austenite
Rapid quenching of austenite traps carbon in a supersaturated body-centered tetragonal structure, producing hard, brittle martensite through a diffusionless transformation.
Question 3: Which heat treatment reduces brittleness of quenched (martensitic) steel by heating below the eutectoid temperature?
- Normalizing
- Full annealing
- Tempering (Correct answer)
- Case hardening
Correct answer: Tempering
Tempering reheats martensitic steel to a sub-eutectoid temperature, allowing some carbon precipitation and relieving internal stresses to improve toughness.
Question 4: The lever rule in a two-phase region of a phase diagram is used to determine:
- The liquidus temperature at a given composition
- The relative weight fractions of each phase present (Correct answer)
- The diffusivity of solute atoms in the matrix
- The equilibrium crystal structure of each phase
Correct answer: The relative weight fractions of each phase present
The lever rule uses the tie line in a two-phase region to calculate the weight fraction of each phase present at a given overall composition and temperature.
Question 5: Precipitation hardening (age hardening) increases strength by:
- Forming a complete solid solution that distorts the lattice uniformly
- Generating fine precipitates throughout the matrix that obstruct dislocation motion (Correct answer)
- Increasing grain size to reduce total grain boundary area
- Dissolving interstitial impurities into the crystal lattice
Correct answer: Generating fine precipitates throughout the matrix that obstruct dislocation motion
Fine, coherent precipitates act as obstacles that dislocations must either cut through or bypass (Orowan looping), both requiring additional stress.
Question 6: The glass transition temperature (Tg) of a polymer is best described as the temperature:
- At which the polymer melts and flows as a viscous liquid
- At which the polymer undergoes chemical decomposition
- Below which the polymer becomes glassy, stiff, and brittle (Correct answer)
- At which crystallization from the melt begins
Correct answer: Below which the polymer becomes glassy, stiff, and brittle
Below Tg, polymer chain segments lack sufficient thermal energy to rotate or translate, making the material glassy and brittle rather than rubbery.
Question 7: Charpy and Izod impact tests primarily measure a material's:
- Surface hardness
- Ultimate tensile strength
- Impact toughness (energy absorbed before fracture) (Correct answer)
- Fatigue endurance limit
Correct answer: Impact toughness (energy absorbed before fracture)
Both tests strike a notched specimen with a swinging pendulum and measure the energy absorbed during fracture as a measure of impact toughness.
Fundamentals of Engineering Materials Science 2
On the iron-carbon phase diagram, the eutectoid composition is approximately: